1. Field of the Invention
The present invention relates to a light guide plate having a light exit surface, a back surface opposed to the light exit surface, and a light entrance surface consisting of at least part of side surface extending between the light exit surface and the back surface, and more particularly to a light guide plate having excellent optical properties. The present invention also relates to a method for producing a light guide plate having excellent optical properties, and to a surface light source device and a display device, both having a light guide plate having excellent optical properties.
2. Description of Related Art
A surface light source device for illuminating a liquid crystal display panel from the back is in widespread use (see e.g. JP 2007-227405A, JP 2008-296587A and JP 9-145931A). Surface light source devices are roughly classified into direct-light type devices which have a light source disposed right behind an optical member, and edge-light type devices which have a light source disposed lateral to an optical member. Edge-light type surface light source devices have the advantage that the surface light source devices can be made thin as compared to direct-light type surface light source devices.
In an edge-light type surface light source device, a light guide plate is provided lateral to a light source, and light from the light source enters the light guide plate from the side surface (light entrance surface) of the light guide plate. The light that has entered the light guide plate repeats reflection at a pair of opposing main surfaces and travels in the light guide plate in a direction (light guide direction) almost perpendicular to the light entrance surface. The light traveling in the light guide plate, by the optical action of the light guide plate, is caused to gradually exit the light exit surface as it travels in the light guide plate. The amount of light, exiting the light exit surface of the light guide plate, can thus be equalized. In an exemplary light guide plate, a light scattering component is dispersed in the light guide plate so that light, traveling in the light guide plate, is allowed to gradually exit the light guide plate.
In addition to the equalization of the in-plane distribution of the amount of exiting light, a surface light source device is also required to increase the front-direction luminance, the efficiency of the use of source light, etc. A common surface light source device (see e.g. JP 2007-227405A), therefore, has various optical members, such as an optical sheet (prism sheet) for adjusting the angular distribution of luminance for a light component traveling along the light guide direction, an optical sheet (prism sheet) for adjusting the angular distribution of luminance for a light component traveling along a direction perpendicular to the light guide direction, etc., disposed on the side of the light exit surface of a light guide plate. It is very desirable if excellent optical properties can be imparted to a light guide plate and therefore the number of optical members in a surface light source device can be reduced. This makes it possible to reduce the production cost of the surface light source device, facilitate the assembly of the surface light source device and make the surface light source device thinner.
On the other hand, a conventional light guide plate is produced by injection molding or thermal transfer processing as disclosed in JP 2007-227405A, JP 2008-296587A and JP 9-145931A. As display devices are becoming thinner with larger screens these days, it is becoming difficult to produce a light guide plate by a conventional method even with such accuracy as to allow the light guide plate to fully perform its intended function. Thus, it is practically impossible to impart additional optical properties to a light guide plate produced by a conventional production method.